1. Field of the Invention
The present invention relates in general to disk storage systems and more particularly, to a method for characterizing a read head based on the correlation of write-induced magnetic read center shifts and read sensitivity changes in the read head.
2. Description of the Related Art
Disk drives are magnetic recording devices used for the storage of information. The information is recorded on concentric tracks on either surface of one or more magnetic recording disks. The disks are rotatably mounted to a spin motor and information is accessed by means of read/write heads that are mounted to actuator arms which are rotated by a voice coil motor. The voice coil motor is excited with a current to rotate the actuator and move the heads. The read/write heads must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information.
Dual element transducers are being increasingly utilized in hard disk drives because they have greater aerial densities than single element transducers. Dual element transducers include a single inductive write element and a separate read element which is constructed from a magneto-resistive material. Such dual element transducers are commonly referred to as magneto-resistive (MR) heads.
The magnetic material in the read or write element of an MR head is generally characterized by a relative permeability .mu., where .mu..mu..sub.0 =B/H, .mu..sub.0 being the magnetic constant (4.pi..times.10.sup.-7), B being the magnetic field and H being the magnetic field intensity. However, magnetic materials are composed of individual domains with local magnetizations equal to the saturation magnetization of the material. When a head is used for writing or reading, the rotation of magnetization within these domains, or the shift of domain walls, constitutes the head response to magnetic fields.
Where there are multiple domain configurations in the head, the domain configuration in an MR read element can be reset from one state to other states by a write field excitation provided by a write element of the MR head. Such changes in the domain configuration of an MR read element often results in changes in readback characteristics of the head, for example, the amplitude, pulse width and shape, as well as the amplitude asymmetry of the readback signal. It has also been observed that the read center for an unstable MR head shifted frequently and randomly from its original position to different positions after write operations. Similarly, when a gated write excitation is applied to an unstable inductive head, its magnetic read center also shifts. Such magnetic read center shifts can cause track misregistration in disk drives. The problem is compounded in disk drives with high storage density. It is thus important to accurately characterize and determine the instability of read heads.
One current technique of characterizing instability of read heads is the use of a coefficient of variance of the track average amplitude (TAA), COV(TAA), which is expressed as follows: EQU COV(TAA)=.sigma.(TAA)/Avg(TAA).times.100%
where .sigma.(TAA) is the standard deviation of the TAA and Avg(TAA) is the average value of the TAA, where each TAA is measured after a write excitation.
However, several factors attributing to the instability are not considered in the above approach. Changes in readback amplitudes for the MR head are due to the combined effects of the read sensitivity change and the magnetic read center shift of the head. FIG. 1 illustrates the standard deviation of the magnetic read center shift under gated write excitations vs. COV(TAA) for a group of MR heads. The average write and read widths for these heads were 3.3 .mu.m and 2.5 .mu.m, respectively. As shown in FIG. 1, the correlation between COV(TAA) and the standard deviation of the read center shifts, .sigma.(shifts), is quite poor. Thus, the use of COV(TAA) to determine the instability of read heads, as well as its use to determine write-induced track misregistration for disk drives is inaccurate and can be misleading.
Accordingly, there is a need in the technology for a simple and accurate method for characterizing a read head based on write-induced magnetic read center shifts and read sensitivity changes in the read head, which will facilitate reduction of track misregistration, as well as the classification and qualification of read heads.